1. Field of the Invention
The present invention relates to a cerium-containing magnetic garnet single crystal, and a production method therefor, and particularly to a cerium-containing magnetic garnet single crystal used as a photomagnetic optical element material and a magnetostatic element material, for example, for optical communication and optical recording of an optical isolator and the like, and a production method therefor.
2. Description of the Related Art
Conventionally, materials for an optical isolator, an important device for optical communication, are required to have low light absorption in the wavelength band used, a large Faraday effect and low change with temperature. As materials satisfying these conditions, iron-containing magnetic garnet single crystals, for example, yttrium.iron.garnet single crystals (Y.sub.3 Fe.sub.5 O.sub.12 : referred to as "YIG" hereinafter) have previously been developed. Such YIG is an incongruent melting type compound, and a YIG single crystal cannot be obtained directly from a raw material having a coincident melt composition. Therefore, the YIG single crystal is generally produced by a flux method.
In addition, since the YIG single crystal has drawbacks with respect to a small angle of Faraday rotation and the high dependency of the angle of Faraday rotation on temperature, the Y site is replaced by Bi as a measure against the small angle of Faraday rotation, and the Y site is replaced by a rare earth element such as Gd or the like as a measure against the high dependency on temperature.
It has recently been found that replacement of y.sup.3+ of the YIG single crystal by Ce.sup.3+ produces a large Faraday effect. However, the sputtering method of production of a magnetic garnet single crystal in which the Y site is replaced by Ce (abbreviated to "Ce:YIG" hereinafter) has the problem that only a film having a thickness of about 1 to 2 .mu.m can be obtained, and such a film is insufficient for use as a material for optical communication and as an electronic device.
In order to obtain a thick film of Ce:YIG, therefore, an attempt has been made to produce Ce:YIG by the same method as a magnetic garnet material in which Y is replaced by Bi and Gd (abbreviated to "BIGd:YIG" hereinafter) and which is used as a conventional isolator material. The method of producing BiGd:YIG is described below.
YIG is known as a so-called incongruent melting type compound and is decomposed, as the temperature is increased, at about 1570.degree. C. Even when a YIG melt having a stoichiometric composition is solidified, YIG cannot be obtained, but a mixture of orthoferrite and iron oxide is obtained. Accordingly, in order to obtain the BiGd:YIG single crystal, a flux method, a floating method, a liquid phase epitaxy method or the like is employed. For example, in the flux method, ferric oxide (Fe.sub.2 0.sub.3), yttrium trioxide (Y.sub.2 O.sub.3), bismuth trioxide (Bi.sub.2 O.sub.3) and gadolinium trioxide (Gd.sub.2 O.sub.3) are dissolved as solutes in a solvent containing lead oxide (PbO) and boron trioxide (B.sub.2 O.sub.3) to prepare a solution. A seed crystal is placed in the thus-prepared solution, followed by gradual cooling to obtain a bulk crystal. The TSFZ method, which is a floating method, produces a bulk crystal in the same manner. On the other hand, in the liquid phase epitaxy method (abbreviated to "the LPE method" hereinafter), a YIG single crystal thin film is grown on gadolinium.gallium.garnet (Gd.sub.3 Ga.sub.5 O.sub.12 : abbreviated to "GGG" hereinafter) to obtain a YIG single crystal film.
In order to obtain a Ce:YIG single crystal having a size sufficient for use as a material for optical communication and an electronic device, the above-described methods are used. However, for example, the LPE method has the problem that the Ce content of YIG is low, and Ce is contained in the crystal as a tetravalent ion, not as a trivalent ion, thereby obtaining no Faraday effect and making the YIG single crystal unsuitable as an optical material due to an increase in light absorption. This is true of the crystals obtained by the flux method and the TSFZ method.
Therefore, only a thin film of a Ce:YIG single crystal can be obtained by the sputtering method, and a Ce-containing magnetic garnet single crystal having a size sufficient for use as a material for optical communication of an isolator and a material for an electronic device cannot be produced.